1. Field of the Invention
The present invention relates to face-centered cubic (fcc) structure alloy particles, and to a method of manufacturing same, for obtaining ferromagnetic particles that can be used in high-density magnetic recording media, nanoscale electronics, permanent magnetic materials, biomolecular marker agents and drag delivery systems (DDS) and the like. More particularly, the present invention relates to non-magnetic fine particles of alloys of platinum group metals (Pt and Pd) and transition metals (Fe and Co) having an fcc structure. These are alloys which are difficult to obtain with the wet method, since it is difficult to simultaneously precipitate both metals from the ions of both metals. This fcc alloy can be heat-treated to phase-transform it to a face-centered tetragonal (fct) structure that exhibits ferromagnetism. That means that the alloy particles of this invention are a precursor for obtaining fct ferromagnetic alloy. As such, the alloy particles of this invention are also called ferromagnetic alloy particle precursors and the like. Also, with respect to the particles of alloy comprised of T and M components expressed by the general formula shown later herein, cited as a typical example is FePt alloy particles in which Fe is the T component and Pt is the M component. While in this specification, the alloy particles of this invention may also be referred to as FePt particles or FePt nanoparticles, these are just examples.
2. Description of the Prior Art
For high-density magnetic recording media to achieve higher recording densities, the size of the recording units has to be reduced. However, with media using a conventional spattered thin film of CoCr alloy, there is a limit on how much the recording density can be increased, due to various problems that include heat fluctuation, refinement or variation of crystal grain size, and so forth. Recently, this has caused attention to focus on FePt magnetic metal nanoparticles, which exhibit high anisotropy and high coercive force, and no heat fluctuation problem even when the crystal grain size is reduced to a size in the order of a single nanometer.
With respect to such magnetic metal nanoparticles, JP3258295B (corresponding to JP 2000-54012A) (Reference 1) and Science Vol. 287, 17 Mar. 2000, pp. 1989-1992 (Reference 2) describe methods of producing FePt alloy particles in a monodispersed state by thermally decomposing iron pentacarbonyl while simultaneously running a platinum (II) acetylacetonate reducing process using polyvalent alcohol.
The FePt particles obtained by these methods have an irregular-phase fcc crystal structure that causes nano-order particles to exhibit paramagnetism at ordinary temperatures. Therefore, to use them as ferromagnetic particles, the irregular phase structure has to undergo transition to a regular L10 fct crystal structure through heat treatment.
This heat treatment has to be done at or above the transition temperature (Tt) at which the crystal undergoes transformation from an irregular to a regular phase structure, which generally is a high temperature of 500° C. or more. During this process, if the grain size distribution is broadened by the heat causing particles to coalesce together into assemblage, the particle structure becomes a mixture of single and multiple domains that is not suitable for high-density magnetic recording media applications. An effective way of preserving the grain size immediately following particle synthesis and obtaining ferromagnetic FePt particles, is to cover the particles with a protective agent that prevents adjacent particles from coalescing, or to use some method or other to reduce the transition temperature to enable the heat treatment to be carried out at a lower temperature.
The Japanese Journal of Applied Physics Vol. 42, No. 4A, 1 Apr. 2003, P. L 350-352 (Reference 3) describes synthesizing FePt particles by the polyol process by using tetraethylene glycol (TEG) as the polyol and reducing platinum and iron acetylacetonate at 300° C., thereby obtaining as-synthesized FePt nanoparticles having an fcc structure.
The Problems to be Solved in the Invention
The FePt particles obtained by the method of References 1 and 2 (also called the IBM method herein) immediately following the reaction have an fcc structure with no magnetism, in which state they cannot be used for high-density magnetic recording media applications. Thus, it is necessary to subject them to heat treatment at or above the fct crystal structure transition temperature for transformation to an fct structure that manifests ferromagnetism. The transition temperature in the case of the FePt particles obtained by the IBM method is in the order of 450° C., so heat treatment at or above 450° C. is required to effect the transition to the fct structure.
However, heating an aggregate of the FePt particles at or above a temperature of 450° C. causes the metal particles to coalesce together into assemblages. Even if an fct structure is achieved, the result is nanoparticles that are unsuited to high-density magnetic recording media applications. Generally, the particles do not coalesce evenly, giving rise to a distribution of grain sizes that produces a major spread in magnetic characteristics that in practice is a problem.
To prevent the particles from being agglomerated by the heat treatment, the particles first have to be spaced apart by a prescribed distance with, for example, each particle being fixed in position on a substrate, or some kind of barrier has to be provided that prevents the sintering of adjacent particles. However, achieving such heat treatment requires the use of precision technology for positioning the particles in a regular arrangement.
In addition, it is difficult to control the particle composition with the IBM method. For example, in a case in which the IBM method is used to fabricate FePt particles comprising 50 at % Fe and 50 at % Pt, for example, the fabrication is only made possible by at least doubling the mole amount of the Fe material. It is still not known what should be done to eliminated variation among particles. As a matter of fact, when the present inventor conducted experiments to corroborate the IBM method and used a TEM-EDS system to analyze the composition of the particles obtained, there was found to be major variations between particles. In the case of FePt alloy, the Pt content in the fct structure that manifests ferromagnetism resides in the range from 35 to 55 at %. Therefore, if there are particles with compositions outside that range, no amount of heat treatment will result in those particles undergoing a transition to an fct structure. Also, even if the Pt is from 35 to 55 at %, if that composition changes from particle to particle, the magnetic characteristics will also change, making the particles unsuitable for high-density magnetic recording media.
Reference 3 shows the possibility of obtaining FePt nanoparticles with an fct structure in the as-synthesized state. However, even when synthesized at 300° C. using TEG, the coercive force Hc of FePt nanoparticles obtained by the method described in the reference is no more than 370 oersted (Oe) at room temperature. When these particles were compared to those fabricated from TEG at 260° C., the possession of an fct structure is confirmed, but a room temperature coercive force Hc of 370 Oe makes them difficult to apply to practical magnetic recording.
An object of the present invention is therefore to resolve the above problem, in particular by improving the method of manufacturing FePt nanoparticles described in Reference 3 by obtaining fcc structure alloy particles (ferromagnetic alloy particle precursor) that makes it possible to obtain magnetic materials constituted by FePt nanoparticles having an fct structure with a small composition distribution that is a suitable material for practical magnetic recording applications.
It was ascertained that the above problem could be resolved by improving the single crystallinity of the fcc structure FePt particles. In the method of manufacturing FePt nanoparticles by the polyol process, in particular, it was found that when FePt particles are synthesized in the presence of a complexing agent, there was low compositional variation of the fcc structure FePt particles and the crystallinity was improved, effectively resolving the above problem.